A coupling for conveying fluid under pressure and elevated temperatures typically has at least one slide-ring seal through which the medium passes between the stationary and the rotating parts and formed of a slide ring and a counter ring sealing against the slide ring.
Slide-ring seals consist of two parts sliding on each other, namely the slide ring and the counter ring. The counter ring is seated in a sealed and axially rigidly connected manner in one of the parts, while the slide ring is installed in an axially freely movable manner and secured against rotation in the other part. The sealing of the slide and counter rings in relation to the adjacent parts is achieved by so-called secondary seals—preferably designed as O-rings.
To move the slide ring axially, the sealing friction force of the secondary seals must be overcome. For the seal to remain closed also in an unpressurized state, springs normally press the slide ring and counter ring together. For simplicity's sake, the spring contact force will not be taken into account in the following. When the seal is traversed by the fluid medium, it penetrates into the seal gap due to the pressure.
The pressure in the seal gap is decreased from the pressurized side, normally in the nonrotating part, toward the downstream low-pressure side, normally in the rotating part. Due to this pressure decrease within or along the sealing surface, there results a force opening the seal gap and separating the seal surfaces. To prevent the gap from opening, a hydraulic load ring bears on the slide ring acts to force closing of the seal gap. The ratio of the gap-closing force part to the gap-opening force part is referred to as load ratio “K.” Material-specific properties of the seal materials and the K value are decisively responsible for the sealing properties and service life of a slide-ring seal.
If a slide-ring seal is operated with a certain medium and permanent operating conditions, such as pressure, temperature and number of revolutions, the K value is configured to the applications correspondingly. For low pressures and low revolutions, load ratios greater than 1 are often used; here, one refers to unbiased slide-ring seals. At higher pressures and increasing RPM, one usually uses biased slide-ring seals with a ratio <1 (conventionally 0.65 to 0.85). A biased slide-ring seal increases the fluid content in the seal gap, thereby better cooling and lubricating the mutually rubbing sealing surfaces, and thus ensuring a correspondingly longer service life of the seal.
If slide-ring seals are used for widely varying fluids and pressures, it is difficult to achieve an optimal configuration for all applications. Such an application of different fluids is found for example when machining on machine tools. There, cooling lubricants are transported directly to the blade through the rotating spindle and the tool installed there. Conducting the medium to the rotating spindle shaft is performed by rotary feedthroughs or seal kits. In these parts, contacting slide-ring seals are used primarily. For cooling the blade, cooling lubricants, multifunction oils, dry compressed air and oil-air aerosols are used depending on the machining step. However, machining without any fluid is also not unknown.
The biggest problem generally occurs when machining with compressed air, in other words without a cooling and lubricating medium, since here there is no lubricant in the seal gap. As a result, the seal becomes hot, wears relatively quickly, and often fails completely due to overheating. To counteract this occurrence, there are various solutions on the market.
In one solution known from actual practice, one alternates between controlled leakage and closed seal surfaces, depending on the pressure and type of medium. However, this has the disadvantage that when supplying the fluid, high leakage rates and undesired energy losses occur when the seal surfaces are open. In addition, dirt particles can enter between the separated sealing surfaces and damage their lapped surfaces when the seals make sliding contact at a later point.
A solution is known from patent EP 1 567 798 [U.S. Pat. No. 7,390,001], in which a thermally controlled coupling creates a force-fitting connection of a sealing ring to an expansion element. When the expansion elements heats up, the change in length is transferred via a coupling to thereby reduce the contact force of the sealing rings, without opening the seal.
In the solution known from JP-A-2010-101361, the seal surfaces are not directly passed through (in the passage between the seal, a tube is installed through which the medium flows), and a load ratio for the slide-ring seal equal to or less than 0.6 is selected. Supposedly, a controlled leak then adjusts itself.
An idea was proposed in JP-A-2008-261405, according to which, for two fluids, different load ratios are achieved at the seal. A smaller K value is hereby provided for compressed air, and a larger K value is provided for a lubricating medium. This is achieved by the displacement of a piston with a hole, so that various ring surfaces are opened or closed. The control piston is moved via an external pressure connection.
A similar solution is proposed in JP-A-2008-64274. For the ring surface to be unbiased for the closing force, a biasing ring surface is also created that is sealed off from the flowing medium and the atmosphere by secondary seals. An external control pressure can reduce the preset compression caused by the structural in the sealing surface correspondingly depending on various fluid and operational modes.